With reference initially to FIG. 1, a conventional catadioptric element or "needle" 10 as formed, for example in the glass of a signalling lamp, is constituted essentially by a plane face for light inlet and outlet and situated on the outside of the glass, plus three plane surfaces 14a-14c each occupying one of the faces of a comer comprising three right angles and situated on the inside of the glass for the purpose of providing retro-reflection by total internal reflection on each of the faces, for incident light that arrives along a direction close to the longitudinal axis of the element.
To ensure that such retro-reflection takes place adequately, the angle a defined by the faces of the corner or "trihedron" when taken in pairs is equal to 90.degree., and these faces are tangential to a cone having a half-angle at the apex .beta. that is equal to 35.degree.15'.
A well known pseudo-catadioptric element 20 is shown in FIG. 2. The faces 24a-24c of its trihedron are disposed relative to each other at an angle .alpha.' which is not equal to 90.degree., e.g. being about 92.degree., such that the half-angle at the apex .beta.' is now equal to 36.degree.25', for example, which means that the retro-reflection function is no longer provided even though the element retains an appearance entirely similar to that of a genuine catadioptric element so long as the lamp is out.
Another known solution for making a pseudo-catadioptric element consists in retaining the shape of the catadioptric element of FIG. 1, and frosting its faces 14a-14c to prevent total reflection taking place thereon.
Both of those known solutions suffer from a first drawback whereby non-catadioptric elements are at least as time consuming and expensive to manufacture as catadioptric elements, even though they have no optical function to perform.
Furthermore, the above-described non-catadioptric elements suffer from the drawback of not passing light emitted from inside the lamp, so that such lamp cannot contribute to the signalling beam that is to be formed.
A known solution to this problem of loss of light is shown in FIG. 3. It consists in providing an intermediate screen E on the inside of the glass G of the lamp, which screen comprises lenses L or the like for concentrating the majority of the light from the source or from a reflector towards zones 30 of the glass that are not subjected to non-catadioptric treatment, and which are thus suitable for passing the light.
That known solution still suffers from the drawback of the lamp costing extra, and the lamp is difficult to bring into compliance with photometric regulations.
A first object of the present invention is to propose a glass that includes non-catadioptric elements that are extremely simple and cheap to make.